1. Field of the Invention
The present invention relates to a heating furnace and more particularly, a heating furnace most suitable for heat treatments such as the thermal diffusion and the CVD (chemical vapor deposition) applied to semiconductor wafers (which will be hereinafter referred to only as wafer or wafers) and the like.
2. Description of the Related Art
The heating furnace is used in the process of manufacturing semiconductor devices. The heating furnace is intended in this case to apply heat treatments such as the thermal diffusion and the CVD to wafers. The temperature in this heating furnace is likely to change every time when the wafers which are to be treated come in and out of the furnace. Therefore, a plurality of wafers are mounted on a wafer boat. This wafer boat is inserted into the furnace and the plural wafers are heat-treated at the same time. The influence which is exerted on the wafers by the temperature change in the furnace is made as trifling as possible by this process. As the result, the throughput of the heat treatment applied to the wafers is enhanced. The temperature in the furnace is usually set from several hundreds to thousand and several hundreds degrees.
As described above, the heating furnace is asked to have temperature uniform in the whole furnace. The conventional heating furnace has the following arrangement, for example, for this purpose. The core tube is made of a material such as quartz. The soaking tube is arranged round the core tube. The heater is arranged round the soaking tube. Layers of heat insulating material are arranged to enclose the heater. The heat insulating material is porous, for example.
In other word, the heater is enclosed by the insulating material, low in thermal conductivity, in the case of the conventional heating furnace. Heat discharge from the core tube to outside is thus reduced and made uniform. As the result, temperature is made uniform in the core tube.
In order to apply uniform heat treatment to the wafers, however, it is needed that temperature change is held .+-.0.5.degree. C. in the whole furnace. However, the above-described conventional heating furnace could not have such a high uniform temperature set inside it.
The heater which is a component for forming the conventional heating furnace comprised turning its heat transfer line round the core tube like a coil along the longitudinal direction of the core tube. However, it is difficult for this heater to carry out uniform heating in the core tube over the whole length thereof.
Therefore, the heater is divided into three, that is, left, center and right zones. Each of these zones is provided with a temperature control means. Further, the turning density of the heat transfer line is made high at those portions thereof which correspond to both ends of the core tube. Heat discharge to the outside of the core tube is thus restrained (see Japanese Utility Model Publication Sho No. 54-43650).
When the heater is arranged in this manner, the uniformly heating zone formed in the core tube is extremely shorter as compared with the whole length of the core tube. This forces a plurality of wafers to be heat-treated in the short heating zone. As the result, many wafers cannot be uniformly heat-treated at one time. The throughput is thus lowered.
Preliminarily-opened Japanese Utility Model Application Sho No. 60-25140 discloses a heating furnace wherein the heater is divided into five zones along the longitudinal direction of the reaction tube and a certain voltage is applied to every zone. The uniformly heating zone can be made longer as compared with that in the heater which is divided into three zones. However, the cost of manufacturing this heating furnace becomes higher because each of the five zones needs the temperature control means.